Dihydroselenapyrans by [4 + 2] Cycloaddition of Diaryl Selenoketones

The phosphorus ylides Ph 3 PCAr 1 Ar 2 5 [Ar 1 = Ph, Ar 2 = 4‐CH 3 C 6 H 4 ( c ); Ar 1 = Ar 2 = Ph ( d ); 4‐ClC 6 H 4 ( e ); 4‐FC 6 H 4 ( f ); 3‐CF 3 C 6 H 4 ( g ); Ar 1 = Ph, Ar 2 = 4‐ClC 6 H 4 ( h )] were allowed to react with elemental selenium at ca. 75°C in toluene in the presence of an excess of 2,3‐dimethylbutadiene. The diaryl selenoketones 1 thus generated in situ by means of the „Staudinger chalcogenation” reaction were trapped by the added conjugated diene to give the 2,2‐diaryl‐3,6‐dihydro‐4,5‐dimethyl‐2 H ‐selenapyrans 7 in high yield. Similarly, the ylides 5c–f and 5h were treated with selenium, and the resulting diaryl selenoketones 1 added to 1,3‐butadiene to give the corresponding 2,2‐diaryl‐3,6‐dihydro‐2 H ‐selenapyrans 8 . Selenobenzophenone, synthesized analogously, was employed in a [4 + 2] cycloaddition reaction with 2,3‐dimethoxybutadiene to yield 3,6‐dihydro‐4,5‐dimethoxy‐2,2‐diphenyl‐2 H ‐selenapyran (9), which was characterized by an X‐ray crystal structure analysis. Compound 9 crystallizes in the space group P 2 1 / n . In the crystal the dihydro‐2 H ‐selenapyran adopts a distorted half‐chair conformation.